Known concrete transit mixers are typically constructed by mounting a concrete mixing assembly to a truck chassis. The mixing assembly includes a concrete mixing drum rotatably supported by front and rear drum support pedestals. These pedestals are fixedly connected to a rectilinear bracket, which is in turn used to secure the assembly to the truck chassis.
Most transit mixers include an open mouthed mixing drum, typically inclined rearwardly at an angle of approximately 14 degrees to the horizontal. There are generally two helical mixing blades 180° apart mounted to the interior surface of the drum. Each blade is similar to an Archimedean screw, except that the pitch and height of the blade often varies along the length of the drum. Normally, the blades have a close pitch and relatively low height near the mouth of the drum and develop continuously in a clockwise direction (or right handed sense) to a relatively large pitch and greater height as the drum diameter increases toward the closed front end adjacent the drum head. The term “clockwise” (and anti-clockwise) when used in conjunction with mixing blade in the context of this application refers to the direction that the drum of the transit mixer is rotated to cause the mixing blades to mix the material in the drum by driving the material forward towards the drum head when viewed from the drum head looking back towards the open end of the drum. As is discussed in more detail below in those countries where vehicles drive on the left hand side of the road clockwise blades are the norm, whereas in right hand drive countries anticlockwise blades are used. The drum is usually rotatably driven about its inclined axis by an hydraulic motor or internal combustion engine connected to the drum head. Existing drums built in Australia usually have a maximum diameter of 2300 mm.
When dry batch ingredients such as sand, stone and cement are fed into the mixer through the open mouth with a suitable quantity of water, the helical mixing blades tend to wind the material progressively forward towards the drum head. During this movement, in prior art transit mixers the helical blades propel the ingredients downward toward the drum head. As the material cannot proceed any further, it accumulates and builds up towards the upper region of the drum head and then folds over itself and through the force of extrusion is driven back toward the rear opening. As the mixing volume is usually about 60% of the total drum volume, the material moving toward the opening slides downward and becomes re-engaged with the lower blades and is once again wound downward toward the drum head. The continuation of this action at about eighteen revolutions per minute for many minutes finally achieves mix uniformity and provides the slump required by the customer.
The mixing time, or total number of revolutions of the drum required to mix the ingredients uniformly, is dependent on a number of factors including:—
(1) The mixing speed;
(2) The shape and volume of the mixing drum;
(3) The design of the mixing blades; and
(4) The mix formulae.
In Australia, any truck mounted mixer must comply with Australian Standard AS 1379-1997 in terms of uniformity of mixing. This test is generally referred to as the “mixing efficiency test”. It has been found that if a truck mounted mixer is to achieve the required degree of uniformity in mixing, the blades must not only induce a rotating motion of the concrete mass parallel to the rotational axis of the drum, but even more importantly, an overall end to end mixing action so that the concrete is uniform throughout its mass.
While most conventional transit mixers can achieve the requirements of AS1379-1997, it is commonly accepted that their mixing efficiency is overshadowed by that of heavy duty central drum mixers which achieve the required uniformity within around a third to a quarter of the time. It is also well known that transit mixers require a higher proportion of cement to be added to their mixes in order to achieve the concrete strength usually obtainable by formulations mixed in central drum mixers.
The main explanation for the superior performance lies in the following characteristics of the central drum mixer:—
(1) Shape and larger diameter of the mixing drum;
(2) Larger internal drum volume (typically around 20 m3 for a 6 m load); and
(3) Mixing blade design adapted to lift material from one side of the drum and discharge in a criss-cross cascading action to the opposite side.
The concrete cascading down on itself induces high impact and frictional forces. According to Portland cement experts, and other authorities these forces are capable of crushing residual cement agglomerations and releasing the entrapped cementitious material for hydration. This leads to central drum mixers requiring a lower proportion of cement in their mixes than would be required by transit mixers to achieve the same ultimate concrete strength. There is a long felt need to provide a truck mounted concrete mixer with a mixing efficiency speed and performance comparable with those of a 20 tonne central drum mixer.
As previously mentioned, both Portland cement experts and indeed most authorities recognise the superior mixing efficiency and reduced mixing time in 6 m3 stationary drum mixers compared to transit mixers. Also, many government authorities often specify the use of large central mixers for concrete to be used in critical mass concrete structures.
Portland cement experts have determined that when dry cement powder comes into contact with water there is a natural tendency for cement agglomerations to form. The common use of chemical additives also assists in the creation of myriads of cocoons, which although entrapping thousands of micron sized cement particles, and water, merely float again in the load due to the gentle folding action of prior art transit mixers. The cement in these cocoons thus never becomes hydrated. By contrast, the cascading turbulence, and friction created in a central drum mixer crushes the agglomerations, releasing the entrapped cement for hydration and thereby resulting in higher strengths and shorter mixing times in central mixers. The agglomerations range in size from minute to golf or even tennis ball size. Agglomerations reduce the strength of the concrete and create other problems. In particular, when the concrete is laid and set, if one of the agglomerations is located at an outer edge of the concrete so it is exposed to atmosphere, it may crumbles or be washed out to leave an unsightly hole. If the lump should be located in a beam, it could adversely affect the strength of that beam.
AU 652893 discloses one attempt to overcome the problems of the prior art discussed above. This patent describes a truck mounted mixing apparatus for concrete which includes a wedge shaped “bucket” formation in the vicinity of the drum head located between the drum and the end of the main blade flight. The angle of the wedge is 23°. On rotation of the drum, the bucket is intended to fill with material from the bottom of the drum and elevate that material for discharge towards the top of the drum head thus aiding extrusion towards the open end and helping to promote end to end mixing within the drum. However whilst the apparatus of AU 652893 was found to assist with sloppy (high slump) concrete, the apparatus tending to raise the sloppy concrete to the upper part of the drum head where it subsequently extruded and slid downwards towards the mouth of the drum on top of the material in the drum, it was later discovered that it did not work with high cement content, low slump concrete. It was found that the drier low slump concrete became wedged or jammed in the drum head bucket instead of being discharged towards the top of the drum end, and thus the apparatus did not work as intended. A further problem which was found to arise was that concrete would remain wedged in the bucket during discharge even after the rest of the load had been discharged. To remove the wedged/stuck concrete it was necessary to wet it down after which that part of the load was unacceptable for use in building. Thus the invention which is the subject of AU 658893 has not been commercially successful with low slump concrete.
It is an object of the invention to overcome or ameliorate one or more of the deficiencies of the prior art, or at least to provide a useful alternative.
Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is solely for the purpose of providing a context for the present invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date of each claim of this application.